Background

The research I am currently undertaking is part of my PhD work. The research aims to address some of the questions and concerns when using biomarkers as soil and sediment tracers. Specifically, I am looking at isotopic ratios of carbon-12 (12C) and carbon-13 (13C) found in very long chain fatty acids (VLCFAs) of plant origin and the ability of these ratios to distinguish sediment sources in agricultural watersheds and factors affecting resolution.

The use of compound-specific stable isotopes (CSSIs) in tracing soil and sediment movement is a relatively new approach to apportioning soil sources in watersheds. Gibbs first applied this approach in 2008, and later Blake et al. in 2012; results indicated that the tool is a promising new technique which may complement other tracing methods (e.g. fallout radionuclides (FRNs), geochemistry, etc).

Fig. 1: A simplified approach to source apportioning using CSSIs.

The role of CSSIs
The use of CSSIs adds further refinement to current tracing techniques. Within a watershed, FRNs and geochemistry are able to apportion sediments into various potential source areas, which are often fairly large in scale. The CSSI technique uses biomarkers from the surface vegetation (i.e. VLCFAs are transferred from plants and incorporated into the soil) to determine the potential soil sources. In a cropped agricultural watershed, for example, soil that originated from a wheat field would be expected to show a different 13C:12C ratio than soil from an alfalfa field. Using a Bayesian unmixing model, such as MixSiAR, then allows for apportioning the soil sources based on the isotopic ratios found in the sediment at a deposition site and examing the potential contribution of the isotope ratios from each source. To perform this analysis, VLCFAs are isolated and the isotopic ratios in source and sediment are examined. Figure 1. provides a simplied summary of the approach.

Fig. 2b: Location of the Stepler subwatershed in the STCW in MB.

Fig. 2a: The town of Horsefly in BC.

Sampling was performed in two different locations: the Horsefly River Watershed (HRW; 2011-2013) in central British Columbia (Fig. 2a), Canada, and the South Tobacco Creek Watershed (STCW; 2012, 2013) in southern Manitoba, Canada (Fig 2b). The purpose of the sampling was to determine the variability in the 13C:12C ratio, both spatially and temporally.

Fig. 3: Location of the three main sampling sites in the STCW. The watershed drains from left to right in the image.

The South Tobacco Creek Watershed (STCW)

The STCW provides a unique opportunity to investigate the environmental factors affecting the 13C:12C ratios of plants, and the challenge in determining a "representative sample". One of the subwatersheds in which sampling was performed consists of a series of fields that undergo crop rotation; additionally, the crops are separated according to topography, thereby the hydrogology of each hillslope is isolated. This minimizes the mixing of the CSSI signal significantly. Consequently, both spatial and temporal (changes throughout the growoing season, and between growing seasons) influences on the signal may be observed.

Three main sampling areas are shown in Fig. 3. The Stepler watershed (lower left) is the furthest upstream, and IS1 toward the right is furthest downstream. Sites IS1 and IS3 are both located along South Tobacco Creek, whereas the Stepler watershed sampling location is a series of fields. The transects that have been used for sample collection can be seen in the figures below.

Stepler transects
The images below show the layout of the various transects sampled in the Stepler subwatershed over a two year period.

IS1 and IS3 transects
The images below show the transects used for sampling in the South Tobacco Creek.

Fig. 6: An overview of the HRW. The town of Horsefly is shown on the left. The HRS label indicates the field where core samples were taken, and is approximately 28 km upstream from the town of Horsefly, as the river flows. The HRB label is an upstream control site (approximately 35 km apart) that is mostly influenced by native forest.

The Horsefly River Watershed (HRW)

The HRW sampling serves a different purpose than that carried out in the STCW. Soil samples were collected in one of the agricultural fields over a three year period to investigate the changes in the isotopic signature during that time period and throughout the growing season. Additionally, resuspended sediment samples were collected at varying distances downstream from the sampled field in order to observe the dilution of the CSSI signal with distance from the source. Sample processing is still ongoing.

HRW sampling
The images below show the sampling sites for the HRW.

Introduction

Research is currently be carried out in the South Tobacco Creek (STCW), Manitoba and Horsefly (HRW), British Columbia, Canada, watersheds. The STCW has been well-studied and monitored with a well-documented history, making this an ideal location for evaluating soil and sediment tracing techniques.

I am conducting research in the HRW and STCW under the supervision of my committee members Drs. Ellen Petticrew and Phil Owens (UNBC) and Dr. David Lobb (University of Manitoba) as part of my PhD research in order to evaluate soil and sediment tracing using compound-specific stable isotopes, primarily (13C and 12C). Although CSSIs have been successfully used in a number of areas (such as food webs) coupled with unmixing models in order to assign source nitrogen, carbon, etc., only recently has the possibility of using CSSIs in sediment tracing been investigated. Two papers of note which have looked at this possibility are by Gibbs (2008) and Blake et al. (2012).

	Fig. 1. Overview of the STCW		Fig. 2. Overview of the HRW

An extensive sampling regime has been established in the STCW, which is intended to address questions of natural variability. Part of the sampling regime is shown in Fig. 3. The watershed is divided into subwatersheds, and crops have been planted accordingly. For example, the wehat field and alfalfa/grass fields drain into different subwatesheds. This minimizes the mixing of CSSI signals between the two crop types as effectively as possible. This type of isolation of crops into differing subwatersheds is typical for the STCW for research purposes.

Some of the natural variability the sampling regime in the STCW is attempting to capture includes variability:

• along transects (shown in Fig. 3 as Step 1, Step 2, etc.) for a particular crop type
• between transects of the same crop type
• between transects of differing crop type

The variability the sampling regime is intended to capture includes variability due to:

• moisture regime (water use efficiency)
• orientation to the sun
• differences between crop types
• soil profile
• soil strength

Other points within the watershed and downstream from the subwatersheds in Fig. 3.are also being sampled for differences in crop type i.e. field samples as well as for transport. Sediment samples have been taken at two points downstream from the main sampling site.

	Fig. 3. Partial sampling regime in the STCW		Fig. 4. Partial sampling regime in the HRW

An inset of part of the sampling regime in the HRW is shown in Fig. 4. The HRW is a relatively non-diverse watershed in terms of agricultural activity; the primary agricultural activity is haying. The remainder of the land is mostly mixed temperate forest (e.g. spruce, fir, cedar, etc.). Variability in the CSSI signal due to crop difference is thus expected to be low relative to the STCW.

One of the main reasons for selecting the HRW is the size of the watershed. Resuspended sediment samples have been taken at various points downstream from the sampling sites shown in Fig. 4. at differeing intervals (e.g. 0.5, 1, 2 km, etc.). The furthest point downstream is approximately 28 km from the main sampling site. The purpose is to investigate the transport of the CSSI signal and observe changes in the signal in a large watershed.

This page will be further updated in the future. If you have any questions and wish to contact me, please click on the 'Home' tab, where my contact information can be found. Thank you for visiting my page.

Poster references

• Ainsworth, E. A., and Long, S. P. (2004) What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2, New Phytol., 165, 351-372.
• Badeck, F.-W. et al. (2005) Post-photosynthetic fractionation of stable carbon isotopes between plant organs—a widespread phenomenon, Rapid Commun. Mass Spectrom., 19, 1381-1391.
• Cernusak, L. A. et al. (2009) Why are non-photosynthetic tissues generally 13C enriched compared with leaves in C3 plants? Review and synthesis of current hypotheses, Funct. Plant Biol., 36, 199-213.
• Chikaraishi, Y., and Naraoka, H. (2003) Compound-specific δD-δ13C analyses of n-alkanes extracted from terrestrial and aquatic plants, Phytochemistry, 63, 361-371.
• Farquhar, G. D. et al. (1989) Carbon isotope discrimination and photosynthesis, Annu. Rev. Plant. Physiol. Plant. Mol. Biol., 40, 503-537.
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• Hobbie, E. A., and Werner, R. A. (2004) Intramolecular, compound-specific, and bulk carbon isotope patterns in C3 and C4 plants: A review and synthesis, New Phytol., 161, 371-385.
• Jetter, R., and Kunst, L. (2008) Plant surface lipid biosynthetic pathways and their utility for metabolic engineering of waxes and hydrocarbon biofuels, Plant J., 54, 670-683.
• Körner, C. et al. (1988) A global survey of carbon isotope discrimination in plants from high altitude, Oecologia, 74, 623-632.
• Ohlrogge, J. B., and Jaworski, J. G. (1997) Regulation of fatty acid synthesis, Annu. Rev. Plant. Physiol. Plant. Mol. Biol., 48, 109-136.
• Van de Water, P. et al. (2002) Leaf δ13C variability with elevation, slope aspect, and precipitation in the southwest United States, Oecologia, 132, 332-343.
• Wang, G. et al. (2010) Altitudinal trends of leaf δ13C follow different patterns across a mountainous terrain in north China characterized by a temperate semi-humid climate, Rapid Commun. Mass Spectrom., 24, 1557-1564.
• Warren, C. R., and Adams, M. A. (2000) Water availability and branch length determine δ13C in foliage of Pinus pinaster, Tree Physiol., 20, 637 -643.
• Wei, K., and Jia, G. (2009) Soil n-alkane δ13C along a mountain slope as an integrator of altitude effect on plant species δ13C, Geophys. Res. Lett., 36, 5 PP.
• Wu, Y. et al. (2009) Effects of different soil weights, storage times and extraction methods on soil phospholipid fatty acid analyses, Geoderma, 150, 171-178.
• Xu et al. (2010) An improved method for determining medium- and long-chain FAMEs using gas chromatography, Lipids, 45, 199-208.

A copy of the poster can be found here. Note this poster is subject to copyright.